The starting of a motor vehicle generally represents a difficult operational process, because three essentially independent control sequences must be coordinated with each other with respect to the time sequences and the metering thereof. It involves the build-up of a torque required for the starting process in the driving engine, the engaging of the starting clutch in order to transmit the torque required for the starting process, and the disengaging of the service brake in order to reduce and remove braking torque previously applied. If these three control sequences take place in a poorly coordinated manner, this adversely affects comfort, in particular a starting jerk as the motor vehicle starts to roll, increased wear and tear on the friction elements of the starting clutch, and increased environmental impact, such as increased fuel consumption and increased noise and exhaust gas emissions. Especially critical is the starting of a motor vehicle on a hill or a stretch of an incline, since poorly coordinated control sequences may result in an undesired and possibly dangerous backward rolling of the motor vehicle opposite the desired direction of travel.
In order to relieve a driver of the difficult control task of starting, devices and methods have been proposed and developed, especially for use in heavy-duty commercial vehicles such as trucks and buses, which generally can be referred to as starting assistance and by which coordination of required control sequences takes place largely automatically.
The functional principle of most known starting assistance controllers consists of the brake pressure of the service brake set by way of the brake pedal being initially maintained in a starting mode, which can be activated with a stopped vehicle, a running driving engine, a disengaged starting clutch, an engaged starting gear, and the service brake actuated by way of the brake pedal, without the driver having to actuate the brake pedal again. If the driver releases the brake pedal and actuates the accelerator pedal within a predetermined time span, the engine torque of the driving engine is increased proportionally to the travel distance, and optionally to the control speed of the accelerator pedal, and simultaneously the starting clutch is actuated in the engaging direction.
If in the process a travel distance of the starting clutch is reached in which the measured or calculated clutch torque corresponds at least to the holding torque required for holding the motor vehicle or to a higher torque, then a control signal to release the service brake is emitted and the service brake is released by lowering the brake pressure. The motor vehicle then is set in motion, and the starting mode is ended upon the full engagement of the starting clutch or upon exceeding a minimum drive speed. If the driver does not actuate the brake pedal and/or the accelerator pedal within the predetermined time span, the starting mode is likewise ended and the service brake is released, without a starting process having taken place.
A corresponding method for preventing accidental rolling away of a stopped vehicle is known from DE 102 44 024 A1. According to this method, with an activated holding function, it is provided during a starting process that the service brake is released upon reaching a certain travel distance of the starting clutch, wherein the travel distance in question either is predetermined or derived from the currently required holding torque. The holding torque is determined from the driving resistance, in which the current vehicle weight and the roadway gradient are taken into account. The service brake is thus only released when the starting clutch already transmits such a driving torque that is sufficient for holding the motor vehicle on a gradient. The driving torque transmitted by the starting clutch, which is still increased more during the further course in order to enable a starting acceleration, thus works against the braking torque of the service brake until the full release of the service brake, which results in increased wear on the starting clutch and an uncomfortable starting jerk. If no foot pedal (brake pedal or accelerator pedal) is actuated within a predetermined time period, the service brake is likewise released and the holding function ended.
A similar method for preventing a rolling away of a motor vehicle is described in DE 10 2004 045 434 A1. This known method provides that the service brake is released as a function of the amount of a measured value and/or a stored parameter. The measured value primarily is a value of the drive torque transmitted via the starting clutch and/or the transmission, which is sufficient for a starting process without any backward rolling of the motor vehicle. This parameter preferably is a time period, after the expiration of which the service brake is likewise released, if within this time period no new actuation of the brake pedal has taken place. The release of the service brake occurs in such a way that the brake pressure is reduced from a fixed pressure value with a constant pressure gradient.
Another method for the starting assistance of a motor vehicle is disclosed in DE 10 2004 017 703 A1. In this known method, the coupling torque transmitted from the starting clutch is determined by calculation, and a parking brake, which can either be a service brake or an additional brake device, is released when the coupling torque determined by calculation is equal to or greater than the braking torque set by the driver by way of the brake pressure or the starting torque required for a desired starting acceleration. The release of the parking brake preferably occurs by means of a ramp-like reduction of the brake pressure, which is to say a lowering of the brake pressure with a constant pressure gradient.
Since the braking torque set by the driver by way of actuating the brake pedal and the starting torque required for a certain starting acceleration generally are definitely above the holding torque required to hold the motor vehicle on a gradient, backward rolling of the motor vehicle on a gradient can be reliably prevented. During the engaging of the starting clutch and the disengaging of the parking brake, however, the starting clutch permanently works against the parking brake, which is associated with higher wear on the starting clutch and an uncomfortable starting jerk.
Finally, a method for the starting control of a motor vehicle is also known from DE 10 2005 001 550 A1. Provided in this method is that the degree of engagement or the travel distance of the starting clutch, at which the signal to release the service brake is emitted, is determined from the reaction of the drive train actuators, such as a coupling actuator of the starting clutch, a power actuator of the driving engine, to the travel distance and/or the control speed of the accelerator pedal. The degree of engagement or the travel distance, at which the signal to release the service brake is emitted, can be determined more exactly from the respectively known deviation between the actual values and the target values, for example the travel distance or the control speed of the starting clutch and/or the engine torque or the rotational speed of the driving engine, and thus a larger deviation of the transmitted driving torque from a target value at the time of a completely released service brake can be avoided. The fundamental disadvantage that the signal to release the service brake is only sent when a relatively high torque is already present on the starting clutch, which acts against the service brake that is still completely engaged at this time, however, still applies.
It can be said in summary that in the known methods for the starting control of a motor vehicle, the signal to release the service brake is only issued when the starting clutch already transmits relatively high driving torque, which corresponds at least to the holding torque required at that moment to hold the motor vehicle on a gradient. Thereafter, the engine torque of the driving engine and the coupling torque of the starting clutch are simultaneously increased further in order to enable a starting acceleration, and the braking torque of the service brake is reduced by a decrease of the previously trapped brake pressure. The coupling torque thus works against the braking torque until a complete release of the service brake, which is associated with the known disadvantages.
Also, the amount of the initial braking torque determined by the trapped brake pressure of the service brake and the course of the function, with which the braking torque is reduced after the signal to release the service brake has been issued, are considered only unsatisfactorily or not at all during the engagement of the starting clutch. The known methods, likewise, contain no practical possibility for a gentle start occasionally desired by the driver, or for holding the motor vehicle with the starting clutch during any slight deflection of the accelerator pedal. If the motor vehicle is located on a downgrade propping off in the direction of travel, starting with the known method by way of a controlled release of the service brake by the driver using the brake pedal without prior release of the brake pedal and letting a fixed time period pass, or without a prior actuation of the accelerator pedal, is not possible.